This invention relates generally to memories that use phase-change materials.
Phase-change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure and the crystalline state involves a more ordered atomic structure. Generally, any phase-change material may be utilized; however, in some embodiments, thin-film chalcogenide alloy materials may be particularly suitable.
The phase-change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter or vice versa. In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states in response to temperature changes. The temperature changes may be induced by resistive heating.
In some situations, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance-determined states may be possible, allowing the storage of multiple bits of data in a single cell.
A variety of phase-change alloys are known. Generally, chalcogenide alloys contain one or more elements from column VI of the periodic table. One particularly suitable group of alloys are GeSbTe alloys.
In any memory, it is desirable to pack the individual memory cells as closely as possible. With conventional phase-change memory materials, there is no real way to self-align the trenches that separate adjacent rows of memory cells from one another. Therefore, critical alignment processing may be necessary to accurately space one wordline from the next. Moreover, extra real estate may be needed between adjacent wordlines to make up for any misalignment between the trenches and the adjacent memory cells.
Thus, there is a need for better ways to form trenches for phase-change memories.